1. Field of the Invention
The present invention relates in general to an air duct and, in particular, to an air duct in a motor vehicle heating, ventilating and/or air conditioning system.
2. Description of Related Art
Air ducts are widely applied in the motor vehicle industry, inter alia, in order, for example, to guide air driven by a blower through a heating, ventilating and/or air conditioning system and, subsequently, towards outlets, for example in the region of the dashboard of the vehicle. In addition to noises which originate from the drive device of the blower, the blower wheel itself, or even the vehicle engine, unpleasant noises frequently arise in the air ducts for the vehicle occupants, in particular when the air speed is high. So far, it has been assumed that a duct wall as smooth and continuous as possible is to be provided in order to avoid noises.
Furthermore, use has been made from time to time of what are termed Helmholtz resonators, which render it possible to reduce acoustic vibrations of a specific frequency. Thus, for example, EP-A-0968857 proposes a heating and/or air conditioning system for a motor vehicle with improved acoustic performance, in which are provided at specific locations, at which noises arise particularly frequently, for example, in the transition region of two abutting air ducts, or at locations at which the direction of the moving air is sharply changed, Helmholtz resonators which are to be designed specially for the specific frequency of the sound waves there. Such Helmholtz resonators usually comprise a cavity that is constructed at the duct wall and is separated from the air duct with the exception of exactly constructed small openings.
Alternatively, it is also possible to provide damping materials, for example in the form of foamed plastic, in the opening region of the cavities. When designing such resonators, it continues to be important in any case to influence the flow properties in the bordering air duct as little as possible. Moreover, Helmholtz resonators can contribute to reducing the sound waves present only at a specific frequency, and so it is only individual peaks in the overall frequency spectrum that are sharply variable both as a function of the equipment and as a function of the state that can be reduced.
A further disadvantage of what are termed Helmholtz resonators is to be seen in that not only are the design and construction complex, but the production also involves substantial additional costs, since virtually closed cavities at duct walls cannot be provided easily by injection-moulding technology.
Consequently, there has long been a need for an acoustically improved air duct which can reduce sound waves occurring over as broad a band as possible, and yet is to be relatively easy to design and, above all, easy to produce.
It is therefore the object of the present invention further to develop an air duct known per se, in particular in a motor vehicle heating, ventilating and/or air conditioning system, in such a way that it is of simple design and easy to produce and can at least partially reduce or eliminate the occurrence of undesired sound frequencies over a wide band.
According to the invention, this object is achieved by virtue of the fact that at least one cross-sectional discontinuity is constructed in the case of a known air duct. Specifically, it has been shown surprisingly that contrary to the previous attempts to provide duct walls that are as continuous and smooth as possible, the construction of a cross-sectional discontinuity can contribute to reducing the development of noise and to damping already existing sound waves over a wide band by making use of phenomena which occur as purely acoustic scattering processes at the cross-sectional discontinuity. The cross-sectional discontinuity should extend substantially perpendicular to the flow direction in the air duct. In the case of particularly simple embodiments, the cross-sectional discontinuity is therefore provided substantially perpendicular to the air duct, although more complex shapes which take account of the flow profile in the air duct are also possible such as, for example, a parabolic shape. Different air duct geometries can therefore lead to different cross-sectional discontinuities.
An embodiment particularly advantageous in terms of production engineering can be achieved by constructing the cross-sectional discontinuity by means of a cross-sectional enlargement of the air duct. It is possible thereby to scatter the incoming sound waves of medium to higher frequency at the cross-sectional discontinuity, the sound pressure at the higher frequencies being lowered by interference and damping of higher modes.
In many applications, it is desired for the air duct to have virtually the same cross section at the inlet and outlet ends, and so it can be advantageous that the cross-sectional discontinuity is constructed as an edge of a cavity bordering the air duct. For example, it is possible to provide such a cavity by local thermoforming of the duct wall. Alternatively, it is also possible for such an air duct with bordering cavity to be injection-moulded, since, by contrast with known Helmholtz resonators, the cavity in the air duct opens over a wide area, in particular virtually over its entire extent.
In a preferred embodiment, the cross-sectional discontinuity is constructed at the end of a projection, for example a duct wall projection, which projects into a cavity bordering the air duct. Additional interferences can be utilized for the purpose of noise damping owing to the construction of the cross-sectional discontinuity on a projection. In a particularly preferred embodiment, the projection is substantially aligned with the bordering air duct wall, other configurations such as, for example, inclined inwards into the cavity, possibly being advantageous, however, depending on application.
The cross-sectional discontinuity is advantageously constructed substantially in the middle in a wall of the air duct, since the flow rate is mostly highest there, it also being possible to encounter or represent resonance phenomena and interference more frequently in the middle of the air duct.
It is preferred, furthermore, that the cross-sectional discontinuity extends over at least 50% of the width of the air duct, in particular substantially over the entire width. In any case, the dimensions of the cross-sectional discontinuity should not be too small, in order to avoid the production of whistling noises, which would diametrically oppose the aim of the present invention.
In a preferred embodiment, the cross-sectional discontinuity is optimized with reference to possible turbulences at a separation edge of an air approach surface, and/or an air approach edge is optimized, such that no, or scarcely any instances of turbulence arise in the case of shear layers set up, or when the flow is once again applied to the original dimension of the duct. For example, by rounding the edges it is possible to effect/support a deflection of the air downstream of/at the cross-sectional discontinuity into the duct, or else into the cavity.
Finally, it is preferred that a plurality of cross-sectional discontinuities are provided which are of different width, of different construction and/or with bordering cavities which are differently dimensioned/constructed. It is thereby possible to avoid the sound components, particularly in the medium frequency band from 1000 Hz to 5000 Hz, since the incoming sound waves of medium to higher frequencies are scattered at the respective cross-sectional discontinuities. Various instances of interference and damping of higher modes can lower the sound pressure at the higher frequencies owing to the variability of the plurality of cross-sectional discontinuities and/or the bordering cavities.